Corrosion sensors are used in the detection and monitoring of loss of material, such as the internal surface of a pipeline wall, due to corrosion and/or erosion from interaction between the material and the environment in contact with the material. Such conditions exist in oil or gas pipelines.
Commonly, corrosion sensors use electrical resistance methods to detect loss of material due to corrosion/erosion. Such sensors are described, for example, in U.S. Pat. No. 6,946,855, U.S. Pat. No. 6,680,619, U.S. Pat. No. 6,680,619, US2011/001498, WO01/70003, WO02/39102, WO2005/036152 and WO8303675.
In U.S. Pat. No. 6,946,855 (Cormon Limited), an apparatus is disclosed for monitoring the effect on a material of exposure to a fluid, and thereby monitoring the effect on a section of pipe for carrying the fluid. The apparatus includes a sensor element exposed to the fluid and formed as a ring of the material coaxially mounted within, but electrically insulated from, the section of pipe. Changes in the electrical resistance of the sensor element are monitored. Preferably, the apparatus also includes a reference element electrically insulated from the pipe, electrically connected in series to the sensor element and protected from exposure to the fluid. The elements may both be made from the same material as the pipe and, as they are contained within it, experience the same temperature and pressure variations as the pipe. In this manner a change in the resistance of the sensor element caused by corrosion/erosion by the fluid accurately indicates the degree of corrosion/erosion of the pipe carrying the fluid.
U.S. Pat. No. 6,680,619 (Corrocean ASA) describes a sensor device for registering voltage drops on corrosion exposed structures. The sensor device is coupled to a surface area of the structure to which a voltage is supplied by electrodes causing an excitation current in that area. There are a plurality sensors arranged in a matrix defining measurement points with defined distances, to obtain signals related to the voltage distribution in the surface area as a basis for determining material thickness and/or structure in the measurement area, thereby determining wall thickness and/or the occurrence of corrosion and/or erosion defects.
US2011/001498 (Roxar Flow Measurement AS) describes a method and device for monitoring a zone of a metal structure in terms of its electrical resistance in order to detect possible defects in the structure, by periodically passing current through the zone in different directions while measuring and recording voltage drops in a number of selected unit areas within the zone, and by combining, for each unit area, at least two measured values recorded during at least two measurements made with current passing in different directions, and by comparing values obtained by at least one similarly obtained value made earlier.
WO01/70003 (British Nuclear Fuels Plc) provides a method and apparatus aimed at improving the range of situations to which field signature method based techniques are applicable from measuring corrosion. In particular, WO01/70003 is directed towards situations in which the location requiring investigation has been the subject of pre-existing corrosion. WO01/70003 provides a method of investigating corrosion at a location, the method including providing a mounting unit for two or more electrical contacts, introducing the mounting unit to a replica of the location which has not corroded, measuring the voltage between the two or more electrical contacts at one or more times, the two or more electrical contacts being in contact with a replica of the location being investigated, introducing the mounting unit to the location which may have corroded, measuring the voltage between and/or the variation in the voltage between two or more electrical contacts at a first time and at one or more other times, the two or more electrical contacts being in contact with the location being investigated, passing a current through the replica location at the time of the voltage measurement and through the location at the time of the voltage measurements. The current is provided by a given source for the various voltage measurements. The voltage measurement for the replica location defines the characteristics for an uncorroded location. The voltage measurements for the location indicates the extent of corrosion which has occurred by the time of the first voltage measurement for the location and/or indicates the progress of corrosion between the first time and one or more of the other times of voltage measurement for the location.
In WO02/39102 (British Nuclear Fuels Plc), a method of monitoring or predicting corrosion using a field signature method is provided which is intended to be applicable to non-linear locations, such as bends, junctions and the like. The method includes obtaining information on a relationship which links voltage measurements, obtained for a location, between two or more electrical contacts in contact with the location at a first time and one or more other times when a current is passed through the location, to the loss of material from the location. The information on the relationship is used in a modelling process which includes the generation of a model of the location, two or more points on that location and modelling the values generated for the voltages which will be measured between the two or more points with a current applied to the location at a first and at least at a second time. The model includes a change in configuration of the location between the first time and the second time so as to model loss of material from the location. The relationship has the expression of a relationship between a factor relating to the model voltage values and a factor relating to the change in configuration and/or location.
In WO2005/036152 (General Electric Company), a system and method for monitoring defects in a structure are provided. The system includes a power supply for supplying an electric current to a monitoring area of the structure and a reference. The system also includes a measurement circuit for measuring a potential drop across at least two contact points of the monitoring area and at least two contact points of the reference. The system also includes a processor adapted to determine a ratio of the monitoring area potential drop to the reference potential drop indicative of a percentage change in a thickness of the structure. The method includes the steps of supplying the current to the monitoring area and the reference; measuring a first potential drop across the monitoring area and the reference; and determining the ratio indicative of the percentage change in the thickness of the structure.
WO83/03675 (Sentralinstitutt For Industriell Forskning) relates to monitoring large structures to detect defects, e.g. cracks. An electric current is impressed on a steel structure equipped with contact points between which are measured voltage drops caused by the impressed current. A relatively large number of fixed contact points are used all over the area which is to be monitored. The voltage drops are measured between selected pairs of contact points and these voltage drops are compared with corresponding voltage drops having been measured previously in the same manner when the structure was in an initial condition, preferably without any defects. Such monitoring is mainly applicable on oil drilling rigs and petroleum production platforms for exploitation of oil and gas fields offshore, where it is important that the monitoring can be performed by means of robust and simple devices resistible to the prevailing rough environments.
The sensitivity of such prior art corrosion detector arrangements is limited by various factors. For instance, changes in the temperature in the environment in which the pipeline is situated affect the electrical resistance of the pipe (e.g. the resistance of steel may change by 0.4% per ° C.). In electrical resistance corrosion monitoring systems configured with an element having an exposed surface to the environment and a reference system external to the environment such as the pipeline fluid environment, changes in fluid temperatures significantly limit the accuracy and sensitivity of the monitoring system if the temperature of the pipeline and external reference system differ. To illustrate, a nominal difference in temperature of 0.25° C. between the pipeline and reference system will cause a change in the resistance ratio of 1000 ppm.
Some prior art systems compensate for temperature variations by taking temperature measurements in the vicinity of the pipeline. However, such temperature measurements will not additionally compensate for hydrostatic and thermal stresses induced in pipeline structures which also influence the measured resistive voltages.
Other prior art systems compensate for temperature by having a sample measurement device (which is affected by both corrosion and temperature) and a separate reference measurement device (which is not affected by corrosion, but is at the same temperature as the sample device). However, in practice, it can be difficult to ensure that the temperature of the reference device exactly matches that of the sample device.
The present invention seeks to provide an improved monitoring system which does not require a separate reference measurement device, but is still able to accurately compensate for temperature and stress variations in electrical resistance.